> An examination of 'Dry Box' desiccant storage within the PCB manufacturing environment

The discussion regarding ‘dry box’ or desiccant storage of Moisture Sensitive Devices (MSDs) has increased in recent years with the introduction of new IPC Joint Industry Standards such as IPC/JEDEC J-STD-033a and J-STD-020. More and more resources on the topic, many contradictory, are available over the internet. Manufacturers are becoming aware of the possible benefits of ultra-low humidity storage, however there are many misconceptions and uncertainties regarding the proper use and functionality of these devices, including exaggerated expectations for results.

Dry boxes can be implemented at various points within the manufacturing environment; for medium to long term inventory, or for short term handling on the production floor, and in some cases (with certain conditions) as a process replacement for baking. Bare boards, raw components and partially assembled PCB’s can benefit from ultra-low humidity storage.

Depending on the application (ie; short term, long term, ‘bake-out’), the items stored within, and their condition as they enter dry storage, various loads will be placed on the storage cabinets; however in all cases user expectations will remain the same; short recovery time after door open, and a constantly maintained ultra-low humidity (1 % - 5% RH) environment. Without the proper assessment of the load that will be placed on a particular dry box application, and an examination of the dry box variations available (ie N2 ‘trickle’, N2 Purge, Multi-desiccators, addition of heating, or a combination of any thereof, etc) it’s almost certain that manufacturers will not be satisfied with their observed results, and will not be storing their devices safely.

There are a variety of options for ‘dry storage’ available that will satisfy current industry specifications. Methods commonly in use include but not limited to; MBB (Moisture Barrier Bags), N2 cabinets, dry air boxes and desiccant dry cabinets.

Many PCB manufacturing facilities are already utilizing one or more of the methods listed above. In some cases however the methods chosen are implemented without an evaluation of their actual effectiveness in the specific environment / application for which they were intended. Variables such as; how often will the inventory stored be accessed, are the packaging materials hygroscopic, have the parts been pre-conditioned, are not always fully considered. Equally important is the fact that these storage devices are rarely reviewed or audited for their effectiveness post installation, and commonly are in such disarray that they are known to be less effective than required.

It is not uncommon to see moisture barrier bags remain unsealed for prolonged periods of time on a busy production floor, for silica gel sachets to be unknowingly used ‘past their prime’, or be stored in containers that are not air tight.

The calculation used to determine the volume of Silica Gel required to maintain the required relative humidity level (RH%) within a MBB is based upon an activity level of which the gel is expected to be capable. Improper handling or storage of Silica Gel sachets means that the activity level will be degraded.

Frequently N2 and dry air cabinets are in use with doors that no longer seal well when closed as a result of over use or abuse, causing them to be less effective than required, and driving running costs up. As a result of the passive nature of these ‘devices’ they are rarely monitored, maintained, or evaluated to determine if they are functioning in an adequate manner.

Desiccant dry cabinets although relatively new to the industry have been popular in industrial and main stream Asia for decades, as a result of the humid climate. The wide variety of applications for humidity control products within that part of the world has led to a number of innovations and advances in desiccation technology that allows for the practical adaptation to the electronics manufacturing industry.

Whatever the method chosen, expectations will be the same; the assurance that the prescribed relative humidity level is maintained at all times, affording valuable moisture sensitive inventory the protection required.

Auto-recycling dry cabinets rely only on desiccant to reduce humidity levels, nitrogen is not required for their standard operation. Dry cabinets rejuvenate their desiccant by routinely stopping desiccation of the cabinet, sealing off the desiccant material and turning on a low watt heater embedded within the desiccant in order to expel the captured moisture. The device that stores and refreshes the desiccant is typically referred to as the ‘dryer unit’. During the refreshing process the effectiveness of the desiccant dryer unit will be tested. Trace amounts of moisture will typically escape back into the cabinet, causing a short term spike in humidity levels. How much the moisture levels will increase inside the cabinet and how long it will be before the recycling is complete (allowing the desiccation of the cabinet to restart) relates again to the effectiveness of the dryer technology in question. Some manufacturers report moisture levels increasing by as much as 4% through the duration of the recycling process.

Some cabinet models utilize data collected from closed loop humidity sensors installed inside the cabinet to determine when and how long desiccant recycling times should be, based on a digitally selected user set point. Other manufacturers utilizing simpler analogue control technology commonly “hard-wire” the RH set point at the factory providing a timer circuit to manage the recycling of the desiccant causing a maximum duration recycling each and every time the desiccant is refreshed. A typical desiccant dryer will require approximately 45 minutes every 6 hours to recharge fully saturated desiccant, although some dryer units require as long as 70 minutes to complete their recharging process. It’s important to note that during this time the dryer unit will not be desiccating the cabinet.

With respect to the problem illustrated above in figures 1 and 2, according to the user, their goals were not met. The user is correct in that they had straight forward expectations of a constant 1% RH. This type of expectation is not uncommon, nor is the frustration and disappointment that typically follows when the RH meters regularly display increased levels of humidity, and never seems to stabilize at the set point. However the problem may lie with a less than adequate determination of the load which would be placed on the cabinets.

In order to effectively maintain the desired RH Level consideration must be given to the ‘load’ that a cabinet will be required to handle. To fully evaluate the ‘load’ the following questions should be examined;

Will the cabinet be opened frequently? How many times per day? How many times per hour?
Is the inventory ‘conditioned’ (Dry), or recently exposed to high RH Levels (Saturated)
How long will the inventory remain in the cabinet?
What other packaging materials will be stored in the cabinet?
Will the cabinet be located in an inventory / stores area or on the production floor?
How much inventory will you actually be storing at any given time?
How is the inventory to be presented? (boxes, bags, reels, trays, tubes, mounted on P&P Feeders, etc.
Installation location

When selecting from among the various manufacturers for any dry cabinet application it is important to evaluate just how air tight the cabinet itself is. Most manufacturers will offer data that illustrates how quickly the cabinet environment will degrade during a simulated power outage. This data will give a good indication of the cabinet’s ability to contribute in limiting the load placed on the desiccant dryer.

Equally important as the air tightness of the cabinet is its ability to ‘recover’. Recovery time can be defined as the time required for the cabinet to return to its set point after an introduction of moisture into the cabinet. There is a variety of approaches to minimizing recovery time, some manufacturers employ a variety of methods in order to assist the cabinet in regaining their set point RH levels, among these approaches are the following;

Varying degrees of forced air circulation triggered by the opening of the cabinet doors
Employing one, two or even three dryer units in a single cabinet.
Adding heating elements to cabinet design
A constant slow trickle of N2. (approx. 1/10 standard N2 cabinet volume)
N2 cabinet purge triggered by a micro switch on cabinet door

The addition of any one of the options above or any combination thereof will have an effect on the recovery times that the cabinet will be capable of providing. Of course each of the options comes with an accompanying cost, in some cases a one-time charge, in other cases adding to running costs each month. Some of the options may be added in the field as an upgrade, others will require factory installation and should be requested at the time of cabinet ordering.